HARQ is an error control technique also known as Hybrid Automatic Repeat request, the purpose of which is to guarantee information reliability. In the HARQ, a receiving end firstly performs forward error correction (FEC). If correct modulation still cannot be implemented, a transmitting end is required to retransmit data. The HARQ therefore avoids shortcomings that the FEC requires complicated decoding equipment and that information consistency is poor in Automatic Repeat reQuest (ARQ) and can set a bit error rate of the whole system to a very low level.
Constellation Remapping (CoRe) also plays an important role in high order modulation HARQ. In order not to increase a bandwidth and thereby to increase a data transmission rate, Mth-order quadrature amplitude modulation (M-QAM) solutions are often adopted in a mobile communication system. However, high order modulation itself is a type of unequal error protection modulation. For M>4, respective bits mapped onto M-QAM symbols have different performances of bit error rate (BER). Inner points in the constellation have small energy and are susceptible to fading. Bits that constitute these symbols have poor bit reliability. In contrast, bits that constitute outer points have good bit reliability. CoRe is just intended to avoid the problem that some bits are always subject to fading. Constellation positions corresponding to respective symbols in retransmission are changed so that the bit reliability after demodulation and combination at the receiving end tends to be even and is improved as a whole, thereby improving system throughput.
For a multi-antenna system, in addition to considering the bit reliability for the high order modulation, a characteristic of multi-antenna space diversity should also be utilized in the CoRe. That is, if the second transmission is performed, after multi-antenna bit mapping, the same transmission bit is placed on a different antenna from that in the first transmission.
We now explain this with two-antenna QPSK as an example. It is assumed that the bit sequence to be transmitted is b0b1b2b3 and constellation mappings at the first transmission and the retransmission are performed in accordance with Table 1.
There are 2 approaches for implementing this mapping method. The first one consists in steps of adjusting, based on Table 1, the order of the bits to be sent, and adopting the same constellation in each transmission to modulate the bits and then send them. The second one consists in firstly allocating the respective bits to respective antennas without the need to adjust the order of the bits, wherein such adjustment is performed by adjusting the constellation, wherein the constellation is adjusted based on bit mapping relationship between the two transmissions, and in adopting in each transmission a different constellation to modulate symbols and then send them. The CoRe as described below applies to both of the approaches.
In Table 1, remapping version No. 0 corresponds to the constellation mapping mode at the first transmission, and remapping version No. 1 corresponds to the constellation mapping mode at the second transmission. In the first transmission, the bit transmitted by an antenna 1 is b0b1 and the bit transmitted by an antenna 2 is b2b3. In the second transmission, the antenna 1 transmits b3b2 whose order is opposite to that of the bits sent by the antenna 2 in the first transmission, and the antenna 2 transmits b1b0 and maps it, according to FIG. 1, in the order of b1b0 whose order is opposite to that of the bits sent by the antenna 1 in the first transmission.
TABLE 1QPSK Multi-Antenna RemappingModulation Mapping RulesRemapping VersionSymbol 1Symbol 20b0b1b2b31b3b2b1b0
When the existing method is used for the multi-antenna remapping, only antenna diversity is taken into consideration. Through the CoRe, an antenna different from that used in the first transmission is employed to send the bits. In such CoRe, only diversity is considered, but signals are not constructed from the view of two dimensions of space and time.